1. Field of the Invention
This invention relates to spectral sensitizers useful in moderate pH aqueous-developable and aqueous-coatable photopolymer formulations. More particularly this invention relates to aqueous-soluble aminoketone sensitizers.
2. Background of the Art
Photoinitiators for polymerization reactions ordinarily have an inherent, but limited range of natural radiation sensitivity. Only when the photoinitiators are stimulated by these particular ranges or wavelengths of sensitivity do the photoinitiators display their desired functional ability. It is not feasible and/or economic to attempt to provide different photoinitiators for each different range of wavelengths to which one would desire a polymerizable system to be sensitive. It is therefore better to find ways of changing the wavelength range of sensitivity for a given photoinitiator. Changes in sensitivity can be made by structural alteration of the photoinitiator, but a more accepted and more easily performed method is spectral sensitization of the photoinitiator. This procedure is commonly practiced by finding dyes which absorb radiation in the described range of wavelengths and which activate the photoinitiator after absorption of the radiation. For each photoinitiator system it is difficult to predict which specific dyes will be able to effectively act as efficient sensitizers. Even though some dyes may work with more than one photoinitiator (or other photosensitive systems such as silver halides, tetrahydrocarbyl borates, etc.), there is seldom any certainty that a compound or class of compounds will work with a particular photoinitiator.
Since many spectral sensitizing dyes absorb visible radiation to perform their function, they may be colored and add an undesirable color, tone, or hue to the polymerized material. It is often desirable to provide spectral sensitizing dyes which may be tested to lose their color after they have been used as spectral sensitizers. This can be accomplished in many ways such as thermal bleaching or photo-bleaching of the dyes.
Sensitizers may also be used which merely increase the speed (sensitivity) of the photoinitiators at those wavelengths to which they are naturally sensitive.
Light-bleachable sensitizers for silver halide imaging having solubilizing quaternary nitrogen substituents have been reported in EPO 029,412 (Postle; publication date Nov. 19, 1979). Those sensitizers are based on an aminobenzylidenemalonitrile structure and are not aminoketones. Their utility in inducing free radical polymerization is unknown.
Japanese published applications 63-168403 (Fuji; publication date Jul. 12, 1988) and 63-309502 (Hitachi; publication date Jul. 12, 1988) generically claims photopolymerizable compositions containing aminoketone and ketocoumarin sensitizing dyes. Neither application describes any specific examples which are within the scope of or demonstrate the surprising increases in speed associated with the instant invention.
Aromatic aminoketones having carboxyl and amino substituents are disclosed in and U.S. Pat. No. 4,755,450 (Sanders; Jul. 5, 1988). Although some of these are soluble in developers with a pH&gt;7.5 are not within the scope of the instant invention, and do not teach speed enhancement by incorporation of a water solubilizing substituent.
Benzophenone and thioxanthone photoinitiators having quaternary ammonium substituents have been described in EPO 279,475 (Nicholl et al.; publication date Aug. 24, 1988), EPO 224,967 (Gwane et al., publication date Jun. 10, 1987) and Davies, M. J.; Gwane, G.; Green, P. N.; Green, W. N. Spec. Publ. -R. Soc. Chem. 1987, 64, 184-5. U.S. Pat. No. 4,602,097 (Curtis; Jul. 22, 1986) discloses water soluble photoinitiators with tetraalkylammonium substituents connected through a (poly)ethylene oxide linking group. The abovesaid photoinitiators are not suitable for use with visible wavelength light sources, and are neither aminoketones nor sensitizers.
Specht et al. (U.S. Pat. Nos. 4,147,552, Apr. 3, 1979; 4,289,844, Sep. 15, 1981, and 4,278,751; Jul. 14, 1981) discloses aminoketone-substituted coumarins having a variety of substituents including methylpyridinium and anilinium substituents. Said quaternary ammonium substituted coumarin sensitizers materials do not fall within the formula of the instant invention. Additionally, they achieve water solubility at the expense of photosensitivity (i.e., speed). In contrast, the sensitizers of the present invention achieve water solubility while enhancing the speed.
Adair et al. (U.S. Pat. No. 4,713,312; Dec. 15, 1987) discloses the use of aminoketone-substituted coumarin sensitizers as sensitizers for photosensitive microcapsules. Those material disclosed are essentially those of U.S. Pat. No. 4,147,552, and must be non-aqueous soluble in order to be incorporated into capsules.
Reilly (U.S. Pat. No. 4,576,975; Dec. 18, 1986) describes Michler's ketone analogues with water solubilizing carboxylate substituents. Those sensitizers have different structural formulae from the dyes of the present invention, both in their skeleton and in their solubilizing substituent.
What the prior art has not taught, but this invention teaches is that aminoketone-substituted coumarin sensitizers of the instant invention provide unexpected increases in photosensitivity while achieving aqueous solubility at moderate pH (i.e., pH 5-11) values.